Current law in California, and laws being passed in other regions of the United States and around the world, require that emission reduction equipment incorporated on vehicles be continuously monitored by on-board-diagnostic (OBD) systems. The function of these OBD systems is to report and set fault codes when emission control devices no longer meet the mandated emission levels.
One of the systems to be monitored is the catalytic-converter, which in current automotive applications is used to simultaneously oxidize carbon monoxide (CO) and unburned hydrocarbons (HC) while reducing oxides of nitrogen (NO.sub.x) in the exhaust gas stream of a spark-ignited engine. Sensors applied in these monitoring applications continuously measure gasses associated with the catalytic-converter and ascertain when the conversion efficiency of the system has been reduced to a level where it is no longer in compliance with the mandated levels of exhaust gas pollutants.
Compliance to the currently defined OBD catalyst monitoring requirement can be accomplished by either one of two metrics. In the case of the first metric, the catalyst system shall be considered malfunctioning when its conversion capability decreases to the point that the HC emissions exceed the applicable emission threshold specified as follows. Transitional Low Emission Vehicles (TLEVs) applications shall employ an emission threshold malfunction criterion of 2.0 times the applicable Federal Test Procedure (FTP) hydrocarbon standard plus the emissions from a test run with a representative 4,000 mile catalyst system (125 hours of operation for medium-duty vehicles with engines certified on an engine dynamometer). The emission threshold criterion for Low Emission Vehicles (LEVs) and Ultra Low Emission Vehicles (ULEVs) applications shall be 2.5 and 3.0 times the applicable FTP hydrocarbon standard, respectively, plus the emission level with a representative 4,000 mile catalyst system. Notwithstanding, beginning with the 1998 model year, manufacturers shall phase in an emission threshold of 1.75 times the applicable FTP hydrocarbon standard for all categories of low emission vehicles, which shall not include the emission level with a 4,000 mile catalyst system. The phase-in percentages (based on the manufacturer's projected sales volume for low emission vehicle applications) shall equal or exceed 20 percent in the 1998 model year, 40 percent in the 1999 model year, 60 percent in the 2000 model year, 80 percent in the 2001 model year, with 100 percent implementation for the 2002 model year. The malfunction threshold shall be based on the emission standards to which the vehicle is certified. For LEV applications, hydrocarbon emissions shall be multiplied by the certification reactivity adjustment factor for the vehicle.
Regarding the second metric, the efficiency determination shall be based on an FTP test wherein a malfunction is noted when the cumulative Non-Methane Hydrocarbon emissions measured at the outlet of the monitored catalyst(s) are more than 50 percent of the cumulative engine-out emissions measured at the inlet of the catalyst(s). The catalyst system shall be considered malfunctioning when its conversion capability decreases to the point that the average FTP non-methane hydrocarbon conversion efficiency of the monitored portion of the catalyst system falls below 50 percent.
A prior art scheme uses electrochemical exhaust gas sensors, primarily Heated Exhaust Gas Oxygen (HEGO) sensors, and their switching characteristics to deduce catalyst deterioration. Currently, zirconia based electrochemical exhaust gas sensors are used for both closed loop engine control and catalyst efficiency monitoring. As typically used, the HEGO provides an indication only of whether the equilibrated exhaust is rich or lean of stoichiometric. The primary deficiency of this prior art scheme comes in its application to the OBD systems to detect catalyst efficiency. This approach relies on measuring a ratio of a number of voltage level transitions (switches) of two HEGOs, one placed in front of the catalytic converter and one placed behind the catalytic converter. Contemporary catalytic converters have a significant oxygen storage capacity (OSC) that dampens out the normal air/fuel cycling used in engine controller strategies. Therefore, the HEGO placed in front of the catalyst records a switch every time the exhaust gas moves from either a lean-to-rich or rich-to-lean state. The aft-mounted HEGO sensor however does not record a switch every time the front HEGO sensor switches, because the OSC of the catalyst acts as an integrator, smoothing out the air/fuel oscillations. As the catalyst deteriorates because of aging, the OSC of the catalyst decreases and therefore the aft HEGO sensor records more switches. By monitoring the aft-mounted HEGO and fore-mounted HEGO sensor switching transitions for a long period and ratioing the number of switching transitions, a parameter referred to as the switch ratio is obtained. This switch ratio is a indicator of the OSC of the catalyst. This switch ratio is then used as a diagnostic parameter for determining the hydrocarbon conversion efficiency of the catalyst. The most difficult problem with this technique is that the switch ratio, which is a measure of the OSC, and the hydrocarbon conversion efficiency of the catalyst, do not normally correlate except under severe aging of the catalyst. As a result, this technique has poor resolution with only the ability to determine gross changes in the catalyst's conversion efficiency, and can be prone to misdiagnosis.
Another problem with the switch ratio technique is that it relies on the air/fuel modulations that result from the error in the air/fuel controller. These modulations can change or even be eliminated with advanced controller strategies, leaving no method of catalyst diagnostic with the standard HEGO sensors.
Other catalyst monitors relying on calorimetric or hydrocarbon sensors have been proposed, but these sensors only operate under lean conditions when there is sufficient oxygen to reduce the existing reducing species.
What is needed is an improved system for estimating tailpipe emissions in a vehicle that complies with the OBD requirements over the full FTP cycle including rich air-fuel excursions. The new approach should also have improved accuracy and resolution, and be less complex.